Operational amplifiers and differential amplifiers have been available in the marketplace for a long time. In a typical application an operational amplifier accepts an input voltage referred to a node typically known as ground, and generates an output voltage referred to the same node. On the other hand, a differential amplifier accepts the difference between two voltages at its inputs and produces two output voltages whose difference is proportional to the difference of the input voltages. Ideally, a differential amplifier does not respond to the average value of the two voltages present at its inputs (also known as input common-mode voltage).
Recently, applications have emerged in the marketplace which require that three input voltages be processed to produce three output voltages such that:                1. the voltage difference between any pair of output voltages is proportional to the difference between the corresponding pair of input voltages through the same proportionality constant,        2. the average value of the three output voltages is constant and unrelated to the input voltages.One approach would be to just use three single-ended amplifiers, but this would not maintain the output common-mode voltage constant and unrelated to the input common-mode voltage.        